Path: utzoo!mnetor!uunet!portal!cup.portal.com!R_Tim_Coslet
From: R_Tim_Coslet@cup.portal.com
Newsgroups: comp.lang.modula2
Subject: Re: union types
Message-ID: <4118@cup.portal.com>
Date: 26 Mar 88 01:50:50 GMT
References: <8803221947.AA00248@nrl-iws6.ARPA>
Organization: The Portal System (TM)
Lines: 35
XPortal-User-Id: 1.1001.4086



>I'm trying to build a Modula-2 interface to a C-library an have hit
>a snag.  Can anyone construct a suitable translation of:
>
>    struct obj_name {
>       int obj_type;
>       union {
>          int id;
>          char *path;
>       } obj_id;
>    };

The direct equivalent of the above C in Modula-2 is....

	ObjName : RECORD
		    ObjType : INTEGER;
		    CASE : BOOLEAN OF
		      TRUE : id : INTEGER |
		      FALSE : path : POINTER TO CHAR |
		    END
		  END


This should creat the same data structure (and actually has one less
identifier: obj_id). The key is that the TAG identifier is optional
in the Variant CASE statement (alot of people never notice this!!!).

I have not yet done any Modula-2 programming but I have been using
Pascal for over eight years (the above is also applicable in Pascal).

I verified this against Niklaus Wirth's book:

	Programming in Modula-2		Third, Corrected Edition

Check the syntax diagrams in Appendix 4 (starting on page 189)